Electrocardiography is the process of recording the electrical activity of the heart over a period of time using a set of electrodes placed on the body of a patient. These electrodes detect electrical changes on the skin that arise from the heart muscle depolarizing during each heartbeat. In a conventional twelve lead ECG, ten electrodes are placed on the patient's limbs and on the surface of the chest. The overall magnitude of the heart's electrical potential is then measured from twelve different angles (“leads”) and is recorded over time. The overall magnitude and direction of the heart's electrical depolarization can be measured at each moment throughout the cardiac cycle. The graph of voltage versus time produced by this noninvasive medical procedure is the electrocardiogram (ECG) waveform.